From Wikipedia: El Capitan is a 3,000-foot (1000m) vertical rock formation in Yosemite National Park, located on the north side of Yosemite Valley, near its western end. This granite monolith is one of the world’s favorite challenges for rock climbers.
Climbers typically take 2-3 days to climb this. At night, you can spot a few odd light spots on the rock. Those are climbers taking a nap in some not very comfortable ledge. We stopped to take a look, and my cousins spotted something yellow on the rock, and insisted that was a climber. All I could see was sheer granite!! It is quite a sight, really.